fix(engine): block dirty integration merge landings

Refuse AI merge landing on a checked-out dirty integration worktree by default. This prevents Fusion from stashing/restoring unrelated project-root edits into main and then marking tasks done against contaminated state.

Fusion-Task-Id: FN-5780

packages/core/src/settings-schema.ts

@@ -268,7 +268,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   ],
   prerebaseDivergenceThreshold: 50,
   mergeConflictStrategy: "smart-prefer-main",
-  merger: { mode: "ai", maxReviewPasses: 3 },
+  merger: { mode: "ai", maxReviewPasses: 3, allowDirtyLocalCheckoutSync: false },
   mergeDiffVolumeMinLines: undefined,
   mergeDiffVolumeThreshold: undefined,
   mergeDiffVolumeAllowlist: undefined,

packages/core/src/types.ts

@@ -367,6 +367,12 @@ export interface MergerSettings {
    *  hard-failing (blocking). Default: 3. The reviewer uses the project's
    *  validator/reviewer model lane — there is no merge-specific model setting. */
   maxReviewPasses?: number;
+  /** Dangerous compatibility escape hatch for the AI merge landing path.
+   *  When false (default), Fusion refuses to land an AI merge if the checked-out
+   *  integration worktree is dirty. When true, restores the legacy stash →
+   *  fast-forward → restore behavior for operators who explicitly accept that
+   *  unrelated local edits can be reintroduced after landing. */
+  allowDirtyLocalCheckoutSync?: boolean;
 }
 
 export const AUTO_RECOVERY_MODES = ["off", "deterministic-only", "programmatic", "ai-assisted"] as const;